EUV and X-ray sources of high intensity are applied in many fields, for instance surface physics, materials testing, crystal analysis, atomic physics, medical diagnostics, lithography and microscopy. Conventional X-ray sources, in which an electron beam is brought to impinge on an anode, generate a relatively low X-ray intensity. Large facilities, such as synchrotron light sources, produce a high average power. However, there are many applications that require compact, small-scale systems which produce a relatively high average power. Compact and more inexpensive systems yield better accessibility to the applied user and are thus of potentially greater value to science and society. An example of an application of particular industrial importance is future narrow-line-width lithography systems.
Ever since the 1960s, the size of the structures that constitute the basis of integrated electronic circuits has decreased continuously. The advantage thereof is faster and more complicated circuits needing less power. Typically, photolithography is used to industrially produce such circuits having a line width of about 0.18 μm with projected extension towards 0.065 μm. In order to further reduce the line width, other methods will probably be necessary, of which EUV projection lithography is a prime candidate and X-ray lithography may become interesting for some technological niches. In EUV projection lithography, use is made of a reducing extreme ultraviolet (EUV) objective system in the wavelength range around 10–20 nm. Proximity X-ray lithography, employing a contact copy scheme, is carried out in the wavelength range around 1 nm.
Laser produced plasmas are attractive table-top X-ray and EUV sources due to their high brightness, high spatial stability and, potentially, high-repetition rate. However, with conventional bulk or tape targets, the operating time is limited, especially when high-repetition-rate lasers are used, since fresh target material cannot be supplied at a sufficient rate. Furthermore, such conventional targets produce debris which may destroy or coat sensitive components such as X-ray optics or EUV multi-layer mirrors positioned close to the plasma. Several methods have been designed to eliminate the effect of debris by preventing the already produced debris from reaching the sensitive components. As an alternative, the amount of debris actually produced can be limited by replacing conventional solid targets by for example gas targets, gas-cluster targets, liquid-droplet targets, or liquid-jet targets.
Targets in the form of microscopic liquid droplets, such as disclosed in the article “Droplet target for low-debris laser-plasma soft X-ray generation” by Rymell and Hertz, published in Opt. Commun. 103, p. 105, 1993, are attractive low-debris, high-density targets potentially capable of high repetition-rate laser-plasma operation with high-brightness emission. Such droplets are generated by stimulated breakup of a liquid jet which is formed at a nozzle in a low-pressure chamber. However, the hydrodynamic properties of some fluids result in unstable drop formation. Furthermore, the operation of the laser must be carefully synchronized with the droplet formation. Another problem may arise in the use of liquid substances with rapid evaporation, namely that the jet freezes immediately upon generation so that drops cannot be formed. Such substances primarily include media that are in a gaseous state at normal pressure and temperature and that are cooled to a liquid state for generation of the droplet targets. To ensure droplet formation, it is necessary to provide a suitable gas atmosphere in the low-pressure chamber, or to raise the temperature of the jet above its freezing temperature by means of an electric heater provided around the jet, such as disclosed in the article “Apparatus for producing uniform solid spheres of hydrogen” by Foster et al., published in Rev. Sci. Instrum. 6, pp 625–631, 1977.
As an alternative, as known from U.S. Pat. No. 6,002,744, which is incorporated herein by reference, the laser radiation is instead focused on a spatially continuous portion of a jet which is generated by urging a liquid substance through an outlet or nozzle. This liquid-jet approach alleviates the need for temporal synchronization of the laser with the generation of the target, while keeping the production of debris equally low as from droplet targets. Furthermore, liquid substances having unsuitable hydrodynamic properties for droplet formation can be used in this approach. Another advantage over the droplet-target approach is that the spatially continuous portion of the jet can be allowed to freeze. Such a liquid-jet laser-plasma source has been further demonstrated in the article “Cryogenic liquid-jet target for debris-free laser-plasma soft x-ray generation” by Berglund et al, published in Rev. Sci. Instrum. 69, p. 2361, 1998, and the article “Liquid-jet target laser-plasma sources for EUV and X-ray lithography” by Rymell et al, published in Microelectronic Engineering 46, p. 453, 1999, by using liquid nitrogen and xenon, respectively, as target material. In these cases, a high-density target is formed as a spatially continuous portion of the jet, wherein the spatially continuous portion can be in a liquid or a frozen state. Such laser-plasma sources have the advantage of being high-brightness, low-debris sources capable of continuous high-repetition-rate operation, and the plasma can be produced far from the outlet nozzle, thereby limiting thermal load and plasma-induced erosion of the outlet nozzle. Such erosion may be a source of damaging debris. Further, by producing the plasma far from the nozzle, self-absorption of the generated radiation can be minimized. This is due to the fact that the temperature of the jet (or train of droplets) decreases with the distance from the outlet, resulting in a correspondingly decreasing evaporation rate. Thus, the local gas atmosphere around the jet (or train of droplets) also decreases with the distance from the outlet.
However, many substances, and in particular liquid substances formed by cooling normally gaseous substances, gives a jet or a train of droplets that experiences stochastic changes in its direction from the jet-generating nozzle. Typically the change in direction can be as large as about ±1° and occurs a few times per minute to a few times per second. This comparatively coarse type of directional instability can be eliminated by means of, for example, the method disclosed in Swedish patent application No. SE 0003715-0. However, for some applications, an extremely high flux stability and uniformity is required. One example of an application where a very high degree of flux stability and uniformity is required is in EUV lithography. In particular, this high degree of stability is required in so-called steppers and in metrology and inspection apparatuses. Even though the method as disclosed in the above-mentioned Swedish application is employed, there are still some micro-fluctuations left in the position of the target. This in turn results in a spatial instability at the focus of the laser beam, i.e. at the desired area of beam-target-interaction, which should be as far away from the outlet nozzle as possible for the reasons given above. The spatial instability leads to pulse-to-pulse fluctuations in the emitted X-ray and EUV radiation flux and spatial instability of the radiating plasma.